P-type 4H-SiC Research Articles

Improvement of the Specific Contact Resistance on P-Type 4H-SiC by Using a Highly P-Typed Doped 4H-SiC Layer Selectively Grown by VLS Transport

This work reports on the improvement of ohmic contacts made on heavily p-type doped 4H-SiC epitaxial layer selectively grown by Vapor-Liquid-Solid (VLS) transport. Even before any annealing process, the contact is ohmic. This behavior can be explained by the high doping level of the VLS layer (Al concentration > 1020 cm-3) as characterized by SIMS profiling. Upon variation of annealing temperatures, a minimum value of the Specific Contact Resistance (SCR) down to 1.3x10-6 Ω.cm2 has been obtained for both 500 °C and 800 °C annealing temperature. However, a large variation of the SCR was observed for a same process condition. This variation is mainly attributed to a variation of the Schottky Barrier Height.

Devices based on series-connected Schottky junctions and β-Ga<sub>2</sub>O<sub>3</sub>/SiC heterojunctions characterized as hydrogen sensors

Abstract. Field-effect hydrogen gas sensor devices were fabricated with the structure of a series connection between Schottky junctions and β-Ga2O3/6H-SiC heterojunctions. β-Ga2O3 thin films were deposited on n-type and p-type 6H-SiC substrates by gallium evaporation in oxygen plasma. These devices have rectifying properties and were characterized as hydrogen sensors by a Pt electrode. The hydrogen-sensing properties of both devices were measured in the range of 300–500 °C. The Pt/Ga2O3/n-SiC device revealed hydrogen-sensing properties as conventional Schottky diode-type devices. The forward current of the Pt/Ga2O3/p-SiC device was significantly increased under exposure to hydrogen. The behaviors of hydrogen sensing of the devices were explained using band diagrams of the Pt/Ga2O3/SiC structure biased in the forward and reverse directions.

Sodium diffusion in 4H-SiC

Sodium diffusion has been studied in p-type 4H-SiC. Heat treatments have been performed from 1200 °C to 1800 °C for 1 min to 4 h. Secondary ion mass spectrometry has been used to measure the sodium distribution. We show that sodium has a considerable mobility at 1200 °C in p-type 4H-SiC. On the other hand for sodium atoms trapped at suitable sites the mobility is limited up to 1800 °C. Trap limited diffusion kinetics is suggested and an effective diffusivity has been extracted with an activation energy of 4 eV for sodium diffusion in p-type 4H-SiC.

On the Schottky Barrier Height Lowering Effect of Ti3SiC2 in Ohmic Contacts to P-Type 4H-SiC

In this paper, an experimental investigation into titanium (Ti) / aluminium (Al)-based ohmic contacts to p-type 4H-silicon carbide (SiC) has been presented. Electrical characterisation of the fabricated contacts showed that metal structures with an initial Ti layer yielded the lowest specific contact resistance (ρc), with a mean value of 3.7×10-5 Ω-cm2 being achieved after annealing in argon (Ar) at 1000°C for 2 minutes. Transmission electron microscopy (TEM) analysis illustrated the epitaxial relationship between the 4H-SiC and the as-deposited Ti layer, and, in conjunction with energy dispersive X-ray (EDX) analysis, showed that after annealing a ~5 nm thick layer of Ti3SiC2 was present, epitaxially arranged with the 4H-SiC. X-ray diffraction (XRD) analysis showed that the presence of the Ti3SiC2 metallic phase was more prevalent in the samples with Ti as the initial metal layer annealed at 1000°C, which corresponded with lower specific contact resistance. Fitting of experimental data to a thermionic field emission (TFE) model allowed the Schottky barrier height to be extracted; it was found that the lowest Schottky barrier heights were more prevalent where the most intense Ti3SiC2 phases were observed during XRD analysis.

Au nanoparticles embedded at the interface of Al/4H-SiC Schottky contacts for current density enhancement

Nanostructured contacts, comprised of nanoparticles (NPs) embedded at the interface of contact/semiconductor, offer a viable solution in modification of Schottky barrier height (SBH) in Schottky contacts. The successful performance of devices with such nanostructured contacts requires a feasible selection of NPs/contact material based on theoretical calculations and a cost effective and reproducible route for NPs deposition. Acidification of commercially available colloidal Au NPs solution by HF has been selected here as a simple bench-top technique for deposition of Au NPs on n- and p-type 4H-SiC substrates. Theoretical calculations based on the model of inhomogeneity in SBH (ISBH) were used to make a more appropriate selection of NPs type (Au) and size (5 and 10 nm, diameter) with respect to contact metal (Al). Al/Au NPs/SiC Schottky barrier diodes were then fabricated, and their electrical characteristics exhibited current density enhancement due to the SBH lowering. The source of SBH lowering was determined to be the local electric field enhancement due to NPs effect, which was further investigated using the models of ISBH and tunneling enhancement at triple interface.

In situ monitoring of stacking fault formation and its carrier lifetime mediation in p-type 4H-SiC

Using the fine control of an electron beam (e-beam) in scanning electron microscopy with the capabilities of both electrical and optical imaging, the stacking fault (SF) formation together with its tuning of carrier lifetime was in situ monitored and investigated in p-type 4H-SiC homoepitaxial films. The SFs were formed through engineering basal plane dislocations with the energy supplied by the e-beam. The e-beam intensity required for the SF formation in the p-type films was ∼100 times higher than that in the n-type ones. The SFs reduced the minority-carrier lifetime in the p-type films, which was opposite to that observed in the n-type case. The reason for the peculiar SF behavior in the p-type 4H-SiC is discussed with the cathodoluminescence results.

Enhancement of carrier lifetime in lightly Al-doped p-type 4H-SiC epitaxial layers by combination of thermal oxidation and hydrogen annealing

We investigated the enhancement of carrier lifetime in lightly Al-doped p-type 4H-SiC epilayers (NA ≃ 2 × 1014 cm−3) by postgrowth processing. A carrier lifetime of 2.8 µs in an as-grown epilayer is increased to 5.1 µs by carbon vacancy elimination, i.e., thermal oxidation at 1400 °C for 48 h. It reaches 10 µs by subsequent hydrogen annealing at 1000 °C for 10 min. The carrier lifetime in the as-grown epilayer is also increased to 4.0 µs by only hydrogen annealing. These results suggest that, in addition to carbon vacancy, there is another lifetime killer in p-type SiC, which cannot be eliminated by thermal oxidation but can be passivated by hydrogen annealing.

Detection of minority carrier traps in p-type 4H-SiC

Contrarily to the case of n-type 4H-SiC, very little is known about the presence of minority carrier traps in p-type epilayers. In this study, we performed the electrical characterization of as-grown, electron irradiated, and thermally oxidized p-type 4H-SiC, by using minority carrier transient spectroscopy. Four minority carrier traps are reported in 1.6–2.3 eV energy range above the valence band edge (EV). Particular emphasis is given to the mid-gap minority carrier trap (EH6∕7) and to its correlation to an energetically close mid-gap majority carrier trap (HK4).

Ti/Al/Si Ohmic Contacts for both n-Type and p-Type 4H-SiC

An ohmic contact process by using tri-layer materials for a source contact of a silicon carbide (SiC) metal oxide semiconductor field effect transistor (MOSFET) is proposed. The authors validate its extremely low contact resistance for both n-type and p-type SiC by a simple process. The characteristics of Ti/Al/Si ohmic contacts were measured by using the transfer length method (TLM). We examined the dependence of the contact resistance on the thickness of each layer of Ti/Al/Si. Then, it is found that Ti/Al/Si contacts with an appropriate thickness show excellent ohmic properties for both n-type and p-type SiC. N-type specific contact resistance (ρn) of 3.7 × 10-6 Ω cm2 and p-type specific contact resistance (ρp) of 1.7 × 10-4 Ω cm2 are obtained with Ti (20 nm) /Al (30 nm) /Si (30 nm).

Minority Carrier Transient Spectroscopy of As-Grown, Electron Irradiated and Thermally Oxidized p-Type 4H-SiC

A total of nine electrically active levels have been detected in as-grown and electron irradiated p-type 4H-SiC epilayers. These traps are found in the 0.32-2.26 eV energy range, above the valence band edge (EV). Of these, six are majority carrier traps whereas three are minority carrier traps. We found that thermal oxidation affects the concentrations of two midgap levels, the majority carrier trap, labeled HK4 and the minority carrier trap identified as EH6/7. The analysis of the irradiation energy and dose dependence of the concentration of these two traps, rules out the possibility that they may share the same origin.

Carrier Density Dependence of Fano Type Interference in Raman Spectra of p-type 4H-SiC

We have investigated Raman spectra of p-type 4H-SiC crystals with hole concentration ranging from 3×1016 to 1×1021 cm-3. For folded transverse acoustic (FTA) doublet modes, Fano interference profiles were analyzed, and the frequency shift due to Fano interference was obtained as a function of hole concentration. We demonstrated that the shift of the FTA doublet modes is a quantitative measure for evaluating of hole concentration of p-type 4H-SiC. Spectral features of transverse optical (TO) and longitudinal optical phonon-plasmon coupled (LOPC) modes were also studied for various carrier concentrations. The results show that the full width at half maximum of the LOPC and relative intensity of TO and the LOPC can be used as other calibration measures for hole concentration.

Observation of deep levels and their hole capture behavior in p-type 4H-SiC epilayers with and without electron irradiation

To design SiC bipolar devices, information on deep levels acting as recombination centers is essential. In this paper, we report on the observation of deep levels in p-type 4H-SiC epilayers with and without electron irradiation before and after annealing at 1000 °C. We performed current deep level transient spectroscopy (I-DLTS) for the samples, and the observed deep levels were located near the valence band (with the activation energies less than 0.35 eV) in all the samples. Based on the change of I-DLTS spectra by the electron irradiation or the annealing, we discussed the origins of the deep levels. Then we estimated the time constants of hole capture by the deep levels and discussed the possibility that the observed deep levels behave as recombination centers.

Solution Growth of <i>p</i>-Type 4H-SiC Bulk Crystals with Low Resistivity

P-type 4H-SiC bulk crystals have been grown at a high growth rate of 1.0 mm/h by solution growth using Si-Cr-Al based melt. The crystals grown from solution with an Al content of 10at% show low resistivity of 35 mcm, which is two orders of magnitude lower than commercialwafers (Resistivity: 2500 mcm). The low-resistivity crystals have flat surface and few solvent inclusions. These results indicate that solution growth is a suitable method for fabricating low-resistivity p-type substrates with low cost.

Electrical Properties of Mg-Implanted 4H-SiC

Because Al and B (elements of III group) in SiC are deep-level acceptors and these acceptors cannot reduce the resistivity of p-type SiC very much, Mg (element of II group) that may emit two holes into the valence band is investigated. A p-type 4H-SiC layer is obtained by 1800 °C annealing of the Mg-implanted layer, not by 1600 and 1700 °C annealing. It is found that a Mg acceptor level in 4H-SiC is too deep to determine the reliable density and energy level of the Mg acceptor using the frequently-used occupation probability, i.e., the Fermi-Dirac distribution function. Using the distribution function including the influence of the excited states of the Mg acceptor, therefore, its density and energy level can be determined to be approximately 1×1019 cm-3 and 0.6 eV, respectively. Judging from the Mg implantation condition, the obtained values are considered to be reliable.

On the charge transfer in the “single-sheet graphene-intercalated metal layer-SiC substrate” system

The proposed scheme for the consideration of charge transfer in the three-layer Gr/Me/SiC system (where Gr is a single-sheet graphene, Me is an intercalated metal layer, and SiC is a substrate) contains three stages. At the first stage, a metal monolayer adsorbed on silicon carbide is considered and the charge of adatoms in this monolayer is calculated. At the second stage, the shift of the Dirac point of free-standing single-layer graphene in an electrostatic field induced by charged adatoms of the monolayer is estimated. At the third stage, a weak interaction between Me/SiC and free-standing graphene is included, which allows electrons to tunnel but does not significantly distort the density of states of free-standing graphene. Estimations are performed for n- and p-type 6H-SiC(0001) substrates and Cu, Ag, and Au layers. The charge state of the graphene sheet and the shift of the Dirac point with respect to the Fermi level of the system are calculated. A comparison with the available experimental and theoretical results shows that the proposed scheme works quite satisfactorily.

Alternative method of interface traps passivation by introducing of thin silicon nitride layer at 4H-SiC/SiO2 interface

ABSTRACTAn alternative approach for reduction of interface traps density at 4H-SiC/SiO2 interface is proposed. Silicon nitride / silicon oxide stack was deposited on p-type 4H-SiC (0001) epilayers and subsequently over-oxidized. The electrical characterization of the interface was done by employing metal-oxide semiconductor (MOS) devices, inversion-channel MOS devices and lateral MOS field effect transistors (MOSFETs).

Analysis of the hole transport through valence band states in heavy Al doped 4H-SiC by ion implantation

The temperature dependence of the Hall hole density and the Hall mobility data of heavy doped p-type 4H-SiC(Al) materials obtained by Al+ ion implantation have been analysed in the frame of the charge neutrality condition and the relaxation time approximation. Samples with implanted Al concentrations in the range 1019–1020 cm−3 and 1950 °C/5 min conventional annealing have been taken into account. The reliability of the calculation has been critically discussed by focusing the attention on both the validity limits of the models for the impurity scattering mechanisms and the adopted Hall factor. By introducing empirical mass anisotropy factors, reasons were given in favour of a generalized use of the unique experimental evaluation of the Hall factor reported by the literature for p-type 4H-SiC, assessed for an Al acceptor density in the range of 1.8 × 1015 cm−3–2 × 1018 cm−3. The simultaneous fits of the Hall hole density and mobility data indicate an electrical activation of the Al impurities of the order or higher than 70% and a compensation of about 10% of the Al acceptors.

Photosensitive Properties of ZnO/<I>p</I>-SiC Heterojunction Structures

We investigated the effect of the substrate temperature (T(s)) on the photosensitive properties of ZnO/4H-SiC structures. A ZnO thin film layer was grown on p-type 4H-SiC substrate (0001) by pulsed laser deposition (PLD) deposited at different substrate temperatures (T(s)) of 200, 400, and 600 degrees C, respectively. It was shown that the specific contact resistance of ZnO on p-type 4H-SiC, which was increased from -1.92 x10(-4) to -4.4 omega x cm2 as T(s) increases. On the other hand, the rate of oxygen outdiffusion decreases for the temperature T(s) increase, as observed by transmission line method (TLM) and auger electron spectroscopy (AES) profile. In addition, high photoresponsivity was observed for the ZnO (T(s) -200 degrees C) on p-type 4H-SiC hetero-junction diodes at ultraviolet region (wavelength of -250 nm), comparing to visible (wavelength of -550 nm) and infrared (wavelength of -880 nm). These results may suggest that the ZnO/p-SiC may be applied to optical sensor applications.

Deep ultraviolet photodiodes based on β-Ga2O3/SiC heterojunction

A deep Ultraviolet (UV) photodiode was fabricated using a heterojunction between β-Ga2O3 with a band gap of 4.9 eV, and 6H-SiC with a band gap of 3.02 eV, and investigated its UV sensitivity. A thin β-Ga2O3 layer (200 nm) was prepared on a p-type 6H-SiC substrate through gallium evaporation in oxygen plasma. The device showed good rectifying properties. Under reverse bias, the current increased linearly with increasing deep-UV light intensity. The responsivity of the photodiode was highest to deep-UV light below a wavelength of 260 nm. The photodiode's response time to deep-UV light was in the order of milliseconds.

Phosphorus Doping into 4H-SiC by Irradiation of Excimer Laser in Phosphoric Solution

We developed a method to dope phosphorus into 4H-SiC by irradiating excimer laser light to 4H-SiC immersed in phosphoric acid solution. A KrF excimer laser was used. The surface is slightly ablated by the laser irradiation. However, no amorphous layer is generated near the surface. Phosphorous is introduced at a concentration of over 1020 cm-3 near the crystal surface. The laser irradiation in phosphoric acid solution significantly improves the ohmic contact characteristic between metal and 4H-SiC. Hall effect measurement shows that the irradiation produces an n-type layer at the surface whose sheet carrier concentration is 2.25×1012 cm-2. In addition, we produce a pn junction by irradiating p-type 4H-SiC. The pn junction shows a rectifying characteristic whose on/off ratio is over 8 decades and ideality factor is 1.06.